The present disclosure relates to a semiconductor structure, and particularly to a finFET transistor including vertical source/drain junctions, and a method of manufacturing the same.
Fin field effect transistors can increase the on-current per unit area relative to planar field effect transistors. One type of fin field effect transistors employs a first pad and a second pad that are connected to a plurality of semiconductor fins. The lateral dimensions of the first pad and the second pad along a horizontal direction perpendicular to the direction of the channels in the semiconductor fins is typically greater than the diffusion length of dopants introduced into the first pad and the second pad. For this reason, dopants cannot be effectively implanted through sidewalls of the first pad and the second pad. Instead, dopants are introduced by ion implantation through the top surfaces of the first pad and the second pad.
The variation in the density of implanted dopants with depth from the top surfaces of the first pad and the second pad introduces a vertical dopant concentration gradient in the first pad and the second pad. Further, the vertical dopant concentration gradient causes formation of non-vertical source/drain junctions, and therefore, induces variations in the channel length as a function of depth. For example, the length of the channel at a depth at which the dopant concentration of implanted dopants is at a maximum is shorter than the length of the channel at another depth at which the dopant concentration of implanted dopants is at a value less than the maximum. The channel length variations caused by the non-uniformity of the channel length as a function of depth degrades performance of the fin field effect transistor through a poorly defined turn-on voltage for the fin field effect transistor.